In a pot, add the chopped chocolate and the butter and let it melt.
Put the pot aside to cool.

Meanwhile, separate the egg yolks from the egg whites.

Whisk the egg whites with half of the sugar and the lemon juice.

Beat the egg yolks with the other half of sugar until they double the volume.

Mix the beaten egg yolks with the cooled chocolate.

Incorporate little by little the wisked egg whites into the egg yolks/chocolate mix.

Pour the composition in an oven pan covered with baking paper and let it bake for 30-35 minutes.

When ready let it cool and the spread some cocoa powder above (don't worry about the aspect from the photos below, because when you slice it it will look OK - it's normal because it has no flour nor baking soda).